METHOD AND DEVICE FOR FORMING A PLUG IN A CONDUIT

Abstract: A method for forming a plug in a liquid-filled conduit (2) by local cooling thereof, so that the liquid in the conduit passes from a liquid to a solid state, as well as a device for implementing the method, are disclosed. The invention is distinguished by the provision, on the circumferential surface of a conduit section (2'), of a hose or duct (8, 10) through which flows a cooling medium and which thus forms a cooling coil (6) round the conduit (2). The cooling duct (8, 10) is connected to a cooler (12) having at least a heat-exchanger unit (14), so that the cooling medium circulating in the thus-formed cooling circuit enables controlled cooling of the conduit (2) to a preset temperature which suitably is below the freezing point of the liquid in the conduit (2).

Description: METHOD AND DEVICE FOR FORMING A PLUG IN A CONDUIT This invention relates to a method for shutting off a liquid-filled conduit temporarily. More specifically, the invention concerns a method for forming a plug in a liquid-filled conduit by local cooling thereof, so that the liquid in the conduit passes from a liquid to a solid state. The invention further relates to a device for imple¬ menting the inventive method. Replacing conduits, conduit parts, leaky valves or fittings in a conduit system can be problematic if the system, for one reason or another, cannot be emptied of the liquid contained therein. Thus, for instance, if a district heating system were to be completely emptied in winter, the temperature would drop rapidly at the sub- scribers', e.g. single-dwelling houses, apartment blocks, and industries. In addition, a huge labour input would be required to vent the entire system and the radiators con¬ nected thereto. The processing industry, e.g. the cellulose indus- try, running round-the-clock production offers another example. In such industries, it may take from a few hours to several days to start up the process when there has been a stoppage, and stoppages are therefore avoided as far as possible. The power industry, for instance, is not only restricted by practical and financial considerations but also by extremely high safety requirements, which is par¬ ticularly applicable to nuclear power plants. Even the layman will appreciate that the entire cooling system of a nuclear power plant cannot reasonably be emptied for replacing conduits or fittings, even after the reactor core has been shut off. Thus, there are many areas where conduit systems have to be partly replaced without being emptied and despite the possible occurrence of autocirculation. Con¬ sidering the conduit dimensions commonly used as well as the amount of liquid contained in such systems, it will be appreciated that very special requirements are placed not only on the mode of operation but also on the equip¬ ment to be employed. State-of-the-art techniques are based on the provi¬ sion, upstream and downstream from the conduit section to be replaced, of a box-like and preferably heat-insulated container surrounding the conduit. This basic idea is specifically described in e.g. SE-B-135,309 or US-A 4,433,556. When thus arranged, the box-like container is filled with carbon dioxide ice or dry ice having a tempe¬ rature of -78 β C. It is to be understood that large amounts of carbon dioxide ice are needed for cooling large-diameter conduits filled with room-temperature liquid, a comparatively large amount of heat having to be cooled off. As is well-known, carbon dioxide ice is the solid state of carbon dioxide (CO2), which does not melt at atmospheric pressure but passes directly to a gaseous state. A low carbon dioxide content in the air presents no danger to man, whereas a carbon dioxide content of about 5% provokes a state of unconsciousness and a carbon dioxide content of above 8% causes death. Consequently, this mode of operation can only be used at places with efficient ventilation. Another drawback, and a fairly significant one, is that the relatively large amounts of carbon dioxide ice (in some cases exceeding 10 tonnes) give rise to logistic problems, e.g. concerning delivery reliability and supply rate. Furthermore, carbon dioxide ice is a fairly expensive commodity. Depending on the aimed-at cooling rate, use may also be made of another mode of operation, in which dry ice is "boiled" in ethanol. However, alcohol has the disadvan- tage of a relatively low flash point (12°C in the case of ethanol), which here means tha , under the strict regula- tions currently in force, no sparking operations are per¬ missible. DE-OS-1,525,867 discloses a third mode of operation, in which a cooling liquid is circulated in a circuit. The cooling liquid is fed to a box of the above type, which encloses a conduit section, and the cooling liquid is supplied in a manner to surround the conduit section to be cooled. Thus, the liquid absorbs heat from the con¬ duit section, and hence from the liquid contained there- in, and this heat is removed to be given off in the heat exchanger. After some time, a plug forms inside the con¬ duit and blocks it, thereby enabling the replacement of intermediate fittings and conduit sections. EP-A1-105 155 is concerned with yet another problem, namely how to adapt to different conduit diameters a reusable box or container through which flows a cooling medium. The EP specification proposes that use be made of replaceable inserts of fibreboard, each having a hole of suitable diameter for the conduit. In actual practice, it has, however, been found that such inserts require fairly extensive sealing, since the cross-sectional shape of the conduit may, for reasons of practicality, deviate up to 3% from a perfectly circular shape. The conduit may thus be oval in cross-section. In most cases, the circumferen- tial surface of the conduit also is uneven, e.g. owing to longitudinal weld joints. Irrespective of the fact that each new project requires specially-designed boxes, which may delay the replacement of e.g. a leaking fitting in a nuclear power plant, it will be appreciated that the use of divisible boxes or containers, through which flows a considerable amount of cooling medium in a cooling circuit, increases the risk of leakage from the boxes or containers. Consi¬ dering that aggressive liquids or gases are often used as cooling media in order to produce the aimed-at low tempe¬ rature, a leak may have disastrous consequences. When forming a plug by freezing in one or more parallel con- duits that are relatively close to one another ("twin conduits"), which is a common occurrence in district heating systems, it often is most complicated, and some¬ times even impossible, to arrange a box of the above type at the intended site. Similar problems are faced in, for instance, pipe bends and T- or Y- unctions. It should also be observed that austenitic conduits may be damaged by treatment at excessively low tempera¬ tures, such as freezing by means of carbon dioxide ice or liquid nitrogen. One object of the present invention is, therefore, to provide a method for cooling a conduit section in a simple and safe manner without resorting to the use of large amounts of cooling medium. Another object of the invention is to provide a device for implementing the inventive method. Yet another object of the invention is to provide a device by means of which the temperature can be reduced under controlled conditions. A further object of the invention is to provide a device that can be used for different conduit diameters, parallel conduits, T- or Y-junctions, and so forth. According to the invention, these and other objects are achieved by a method as defined in the characterising clause of appended claim 1, as well as by a device as defined in characterising clause of appended claim 6. Preferred embodiments and improvements of the inven¬ tion are achieved by the features recited in the appended subclaims. Thus, it is advantageous to use a flexible sleeve holding a contact medium which surrounds the con¬ duit section to be cooled as well as the cooling duct wound round the conduit section. An embodiment of the inventive device for forming a plug in a liquid-filled conduit by local cooling thereof, so that the liquid in the conduit passes from a liquid to a solid state, will be described in more detail below with reference to the accompanying drawings, in which Fig. 1 is a schematic view illustrating the use of a device according to the invention; Fig. 2 is a side view of a sleeve used in one embo¬ diment of the invention; and Fig. 3 is a sectional view of the sleeve in Fig. 2, taken along line A-A. Fig. 1 schematically illustrates one embodiment of the inventive device for implementing the method defined in appended claim 1 for forming a plug in a liquid-filled conduit 2. This embodiment includes a heat-insulating sleeve 4 which substantially encloses a cooling coil 6 consisting of a cooling duct 8, 10 which is helically wound round the conduit 2, as well as the conduit section 2' in this area. The feed duct 8 and the return duct 10 connect the cooling coil 6 to a cooler 12. Use can be made of any prior-art cooler having a heat-exchanger unit 14 and a compressor unit 16, e.g. connected to the public power network. It should be mentioned already at this stage that, in some cases, it is advantageous to connect two coolers 12 or compressors 16 in series and/or to connect a spare power installation for reasons that will appear further below. It is to be understood that the feed duct 8 and the return duct 10 may be branched, in which case a single cooler 12 serves to cool another conduit section as well (not shown). Instead of being connected in parallel, the cooling coils 6 may, of course, be connected in series, which is especially advantageous when plugs are to be formed in substantially parallel conduits that are close to one another. Figs 2 and 3 illustrate an embodiment of the inven¬ tive device. As appears from Fig. 1, a cooling coil 6 is formed by arranging a cooling duct 8, 10 on the outside of the conduit section 2'. It is to be understood that, in actual practice, the cooling duct is wound round the conduit. In the area of the cooling coil 6, the cooling duct is made of a material of good thermal conductivity and good flexibility. Outside the area of the cooling coil 6, and in particular outside the sleeve 4 surround¬ ing the cooling coil 6 in the embodiment illustrated, the cooling duct 8, 10 is heat-insulated. The sleeve 4 arranged round the conduit section 2' essentially con¬ sists of an elongate, cylindrical casing having a longi¬ tudinal slot. Lead-ins 20, 22 for the cooling duct 8, 10 are arranged on essentially the same side of the sleeve 4 and preferably consist of recesses formed in the meeting edges of the longitudinal slot 24. As is evident from Fig. 2, the sleeve 4 is further provided with tightening straps 26, 28 for sealing the sleeve 4 round the conduit 2 and along the slot 24. In the radial direction, the sleeve 4 may be com- posed of a plurality of layers 30-36 (see Fig. 3 in par¬ ticular) exhibiting special and appropriate properties. Thus, an inner layer of one embodiment of the inventive sleeve 4 consists of water-tight flexible sheeting 30, e.g. of plastic. An intermediary layer of this embodiment consists of a preferably flexurally elastic, heat-insu¬ lating material 32, such as cellular plastic. Naturally, it is perfectly conceivable to use cellular plastic with completely closed cells, i.e. cellular plastic lacking water-absorbing capacity, in which case the inner layer 30 may, of course, be dispensed with. An outer layer of this embodiment consists of a preferably rigid, albeit flexible material, e.g. aluminium, forming a surrounding shell 34. As appears from Fig. 3, the layers 30-34 have a sub- stantially circular cross-section and are provided with a slot 24 extending in the longitudinal direction of the sleeve 4. Thanks to the longitudinal slot and the flex¬ ible and flexurally elastic materials used, the sleeve can be passed over a conduit 2 or a conduit section 2' surrounded by a cooling coil 6. By tightening the straps 26, 28, the longitudinal edges of the slot 24 are pressed against one another, in some cases to the point of overlapping. Tightening of the straps 26, 28 further pro¬ duces a sealing action between the terminal areas of the sleeve 4 and the conduit 2. As shown in Fig. 3, a gap 36 is formed between the side of the cooling coil 6 facing radially outwards, or the conduit section 2' at least partly surrounded by the cooling coil 6 (see Fig. 1), and the sleeve 4. In a preferred embodiment of the invention, the gap 36 is filled with a contact medium in the mounted state of the sleeve 4. It is to be understood that it is of the utmost importance, in particular in the case of nuclear power plants, that use is not made of any liquids and/or gases that, either separately or in combination with other substances present, might lead to highly exo¬ thermic or pressure-increasing reactions. For this rea- son, water is preferably used as contact medium, having proved to be a good thermal conductor which contributes to the heat transfer from the outside of the length of conduit 2' to the cooling coil 6. In order to reduce thermal losses upstream and downstream from the conduit section 2' when the conduit 2 is being cooled, the con¬ duit 2 is preferably provided with a heat insulation 38, e.g. in the form of plastic sheeting wound round the con¬ duit 2. Another embodiment of the sleeve (not shown) employs a flexible cellular-plastic material also used in divers' dry suits. This embodiment comprises a liquid-tight seal¬ ing means, e.g. in the form of a zip, operable from the two short sides of the sleeve and adapted to seal the longitudinal slot, i.e. seal the sleeve. This embodiment offers many opportunities as to the arrangement of the cooling ducts and is particularly well suited for use in connection with vertical conduits. The sleeve is easily adapted to different conduit diameters with the aid of elongate strips made of the same material, which are disposed in the elongate slot by means of corresponding zip halves, thereby increasing the circumferential extent of the sleeve and thus enabling the enclosure of larger- diameter conduits. As appears from Fig. 3, thermometers are arranged inside the conduit section 2'. Preferably, three ther- mometers 40-44 are positioned on one diameter, namely one thermometer 42 at the centre of the conduit section 2* and one thermometer 40 and one thermometer 44 positioned at the respective ends of the diameter. In addition, the conduit 2 should be provided with a barometer (not shown) for monitoring the pressure increase when a plug 46 is building up in substantially circular fashion from the inside of the conduit towards its centre. It should be observed that the pressure increase may be considerable and even cause the conduit or conduit section to explode. In order to avoid such an occurrence, it may be necessary to reduce the pressure in the conduit. When e.g. a leaky valve or a conduit section 2' is to be replaced, a cooling coil 6 is initially arranged on the conduit 2 in the area where a plug 46 is to form inside the conduit section 2' . The cooling coil 6 may consist of a separate hose or duct which is helically wound round the conduit and whose ends are then connected to, respectively, the feed duct 8 and the return duct 10. Preferably, a heat insulation 38 is then arranged round at least part of the conduit 2 outside the area of the cooling coil. Thereafter, the flexible sleeve 4, which is slit in the longitudinal direction, is mounted. The sleeve 4 is first bent apart and then passed over the conduit 2 and the cooling coil 6, respectively, so as to enclose the cooling coil 6 and a conduit section to the right and to the left of the cooling coil 6. The edges of the slot 24 are then caused to meet or overlap, e.g. by means of the zip, whereupon the sleeve 4 is fixed and its short ends are sealed against the conduit 2 by the tight- ening of belts or straps 26, 28. The ends of the cooling coil 6 that are connected to, respectively, the feed duct 8 and return duct 10 thus pass through lead-ins 20, 22 provided in the sleeve 4. Before sealing the sleeve 4 completely, the gap 36 between the inside of the sleeve 4 and, respectively, the conduit section 2' and the cooling coil 6 is preferably filled with a liquid or gaseous contact medium, such as water, through a closable opening (not shown) provided in the sleeve. Owing to the flexurally elastic cellular- plastic material used for the sleeve 4, there is produced a sealing action between the sleeve 4 and the conduit 2 when the straps 26, 28 are tightened, even if the sleeve is filled with a contact medium or is used in substan¬ tially vertical position. In view of the risk of leaks, the potential danger of explosions, the heat transfer efficiency, and so forth, a single duct is preferably used as cooling duct, in which case different lengths of the duct form the feed duct 8, the cooling coil 6 and the return duct 10, these being connected to a suitable cooler 12. For the same reason, a non-aggressive cooling liquid, such as a water- based saline solution, is preferably used as heat- exchanging medium. A suitable salt is KC1. If the cooler employed has a sufficiently high capacity, several cool¬ ing coils 6 may be connected in series or in parallel and be supplied with cooling liquid from the same cooler. In contrast to the prior art, the interior of the sleeve 4 thus does not form part of the cooling circuit made up of the cooling coil 6, the feed and return ducts 8, 10, and the heat-exchanger unit 14 of the cooler 12. The sleeve is filled with a contact medium which may be separate from the cooling medium circulating in the cool¬ ing circuit and which is intended to contribute to improve, and hence speed up, the heat transfer between the outside of the conduit section 2' and the cooling coil 6. When the contact medium employed is a liquid, such as water, it has been found that the liquid is drawn by capillary action into small cavities found on the cir- cumferential surfaces of the conduit and the cooling coil. Thus, a large contact surface is formed between the circumferential surface of the conduit and the cooling coil, rendering the heat transfer more efficient. Thus, it is particularly advantageous to clean the circumferen¬ tial surface of the conduit at an initial stage, thereby to remove any dirt that might prevent efficient heat transfer. After connecting the compressor unit 16 of the cooler 12 to the public power network and/or to a spare power source, such as a battery unit or a generator, the temperature of the cooling medium drops rapidly to a pre¬ set, low value, owing to the fact that the amount of cooling medium employed is fairly small. The temperature that can be achieved is essentially restricted only by the cooling medium employed, and in practice ranges from about -20 C C to -40 β C. At the same time, heat is trans¬ ferred from the liquid in the conduit 2 to the conduit itself and further to the cooling coil 6 as well as the contact medium in the sleeve 4. Since the contact medium in the sleeve 4 surrounds those parts of the cooling coil 6 that do not come into direct contact with the circum¬ ferential surface of the conduit 2, heat is transferred to the cooling medium in the cooling coil 6 directly through the contact of the cooling coil 6 with the cir¬ cumferential surface of the conduit 2 and indirectly through the contact of the cooling coil 6 with the sur¬ rounding contact medium. After a certain period of time which, depending on, inter alia, the capacity of the cooler 12, the diameter and the wall thickness of the conduit 2, the flow velocity and the heat content of the liquid in the conduit, the cooling medium in the cooling coil, and the contact medium employed, ranges from a half hour or so to several hours, an annular plug 46 begins to build up inside the conduit 2. As the plug inside the conduit builds up, the pres¬ sure in the conduit 2 increases, attaining a maximum value when the plug blocks the conduit completely. Thus, the pressure in the conduit 2 is a reliable indication of the build-up of the plug and the blocking of the conduit. Once the conduit 2 has been blocked, fittings or conduit sections can be removed or replaced. Even though the cooler should operate continuously in order to maintain the temperature of the cooling medium in the cooling coil and, hence, of the plug in the conduit, comparatively long standstills are acceptable. One reason for this is that it takes time before the plug in the conduit loosens owing to the heat insulation provided. Also, the contact medium employed delays melting of the plug in the con¬ duit. However, it will be appreciated that a lengthy power failure, or other such unfavourable events, might have disastrous consequences. It is therefore advisable to connect a spare power unit in the case of a prolonged power failure. The invention thus provides a new method for forming a plug in a conduit by cooling a conduit section, a cool- ing hose or duct being wound round the conduit section where the plug is to be formed. In order to enhance the degree of efficiency of the heat transfer from the con¬ duit to the cooling medium in the cooling coil, a sleeve is provided, which substantially encloses the cooling coil and is filled with a contact medium. By choosing a suitable contact medium, the degree of efficiency of the heat transfer can be enchanced even further. By combining a high-capacity cooler with the inventive cooling duct, it is possible to wind the same cooling hose or duct round several parallel conduits, so that several plugs may be formed at the same time. Safety is extremely high, especially since the cooling circuit is closed and non- aggressive cooling and contact media may be chosen. It goes without saying that those skilled in the art may devise alternative embodiments and other means pro¬ ducing the same technical effect and thus being encom¬ passed by the basic inventive concept. The contact medium employed may thus be a gas, such as air, or a liquid, such as water. Also the sleeve may comprise alternative or additional layers exhibiting special and appropriate properties. The cooling duct may have a special shape in the area of the cooling coil in order to enlarge the con¬ tact surface with respect to the circumferential surface of the conduit. All such alterations and modifications are intended to be encompassed by the scope of protection as claimed.

Claims: CLAIMS 1. A method for forming a plug (46) in a liquid- filled conduit (2) by local cooling thereof, so that the liquid in the conduit passes from a liquid to a solid state, c h a r a c t e r i s e d by the steps of arrang¬ ing, on the circumferential surface of a conduit section (2'), a hose or duct (8, 10) through which flows a cool- ing medium, thereby to form a cooling coil (6) round the conduit (2); connecting the cooling duct (8, 10) to a cooler (12) having at least a heat-exchanger unit (14) and a control unit; and presetting a temperature in the control unit of the cooler, this temperature suitably being below the freezing point of the liquid in the con¬ duit, thereby enabling controlled cooling of the conduit (2) to the preset temperature. 2. A method as set forth in claim 1, c h a r a c ¬ t e r i s e d by the step of arranging round the conduit (2) a heat-insulating sleeve (4) substantially enclosing the conduit section (2' ) and the cooling coil (6). 3. A method as set forth in claim 2, c h a r a c ¬ t e r i s e d by the step of designing the sleeve (4) as an elongate cylinder having a slot (24) extending in the longitudinal direction thereof and provided for mounting the sleeve (4) round the conduit section (2' ) and the cooling coil (6). 4. A method as set forth in claim 2 or 3, c h a r ¬ a c t e r i s e d by the step of filling the sleeve (4) with a contact medium surrounding those parts of the con¬ duit section (2' ) and the cooling coil (6) that do not come into direct contact with one another. 5. A method as set forth in any one of claims 1-4, c h a r a c t e r i s e d by the step of arranging a heat insulation (38) round the conduit (2) on both sides of the conduit section (2' ). 6. A device for forming a plug (46) in a liquid- filled conduit (2) by local cooling thereof, so that the liquid in the conduit passes from a liquid to a solid state, c h a r a c t e r i s e d in that there is arranged, on the circumferential surface of a conduit section (2'), a hose or duct (8, 10) through which flows a cooling medium and which thus forms a cooling coil (6) round the conduit (2), that the cooling duct (8, 10) is connected to a cooler (12) having at least a heat- exchanger unit (14), and that the cooling medium circu¬ lating in the thus-formed cooling circuit enables con¬ trolled cooling of the conduit (2) to a preset tempera¬ ture which suitably is below the freezing point of the liquid in the conduit (2). 7. A device as set forth in claim 6, c h a r a c ¬ t e r i s e d in that a heat-insulating sleeve (4) sub¬ stantially encloses the conduit section (2' ) and the cooling coil (6), said sleeve consisting of an elongate cylinder which has a slot (24) extending in the longitu- dinal direction thereof and provided for mounting the sleeve (4) round the conduit section (2' ) and the cooling coil (6). 8. A device as set forth in claim 7, c h a r a c ¬ t e r i s e d in that the sleeve (4) is provided with a substantially liquid-tight sealing means for sealing the longitudinal slot (24) after the sleeve (4) has been mounted round the conduit section (2' ) and the cooling coil (6). 9. A device as set forth in claim 7 or 8, c h a r - a c t e r i s e d in that the sleeve (4) is made of a flexurally elastic, heat-insulating cellular-plastic material. 10. A device as set forth in claim 9, c h a r a c ¬ t e r i s e d in that the cellular-plastic material has substantially closed cells. 11. A device as set forth in any one of claims 7-9, c h a r a c t e r i s e d in that the sleeve (4) has an inner layer of liquid- and/or gas-tight, flexible sheeting facing the cooling coil (6) and the conduit section (2' ). 12. A device as set forth in any one of claims 7-11, c h a r a c t e r i s e d in that the sleeve (4) has an outer layer of a rigid, albeit elastic, material, pref¬ erably metal. 13. A device as set forth in any one of claims 7-12, c h a r a c t e r i s e d in that means (26, 28) are pro- vided for releasably fixing the sleeve (4) to the conduit (2). 14. A device as set forth in claim 13, c h a r ¬ a c t e r i s e d in that said means consist of tighten¬ ing straps (26, 28) extending circumferentially round the sleeve, so that the edges of the slot (24) meet or over¬ lap as the straps (26, 28) are tightened, thus producing a sealing action between the sleeve (4) and the conduit (2). 15. A device as set forth in any one of claims 7-14, c h a r a c t e r i s e d in that a gap (36), which is defined between the inside of the sleeve (4) and the con¬ duit section (2' ) and the cooling coil (6), is filled with a contact medium in gaseous or liquid state. 16. A device as set forth in any one of claims 6-15, c h a r a c t e r i s e d in that the conduit (2) is pro¬ vided with a heat insulation (38) on both sides of the conduit section (2' ) and the cooling duct outwardly of a sleeve (4). 17. A device as set forth in any one of claims 6-16, c h a r a c e r i s e d in that a pressure transducer is arranged in the conduit (2) for controlling the pressure therein, and that the sleeve (4) has a closable opening through which it can be supplied with a contact medium for more rapid heat transfer to the cooling medium in the cooling coil (6).

Inventors: BEISE LOTHAR, BEISE, LOTHAR
Applicants: BEISE LOTHAR, BEISE, LOTHAR
Application year: 1994
Main classification: F16L55
IPC codes: F16L55